Referring to FIGS. 1 and 1A, a device of this type conventionally comprises a hydraulic manipulator 1 with four hydraulic outlets, respectively denoted by AV for the forward movement control, AR for the reverse movement control, D for the right movement control and G for the left movement control. The manipulator 1 includes four hydraulic pressure reducers 3, respectively denoted by 3 AV, 3 AR, 3 D and 3 G, the outputs of which constitute the four aforementioned outlets of the manipulator 1 and which are furthermore connected to a common pressurized-fluid input P and a common return-to-reservoir output T. The four reducers 3 are arranged in a cross, associated in pairs with two substantially perpendicular directions FORWARD/REVERSE and RIGHT/LEFT, and are in general operated mechanically, via respective sliding tappets (not shown), with the aid of a single oscillating cam (not shown) with a central neutral position N, to which a single actuation lever 2 is attached (FIG. 1A).
A selection device 4 is functionally associated with the manipulator 1 and includes four selectors 5, arranged in a bridge, the pairwise opposite vertices of which are respectively connected to the four outlets AV and AR, on the one hand, and D and G, on the other hand, of the manipulator. The four selectors are respectively denoted by 5 AVG, 5 AVD, 5 ARG and 5 ARD (FIG. 1).
In the conventional set-up adopted usually to date, the respective outputs of the four selectors 5 are connected directly to four variable-capacity motors 6 for directional control of the vehicle, which are denoted respectively by 6 ARG, 6 AVG, for the left running-component control, and 6 ARD and 6 AVD, for the right running-component control, as represented in FIG. 1.
The manipulator is installed ergonomically in the vehicle, that is to say the movement of the actuation lever 2 coincides with the direction of movement desired for the vehicle: the AV/AR movement direction of the lever 2 coincides approximately with the longitudinal axis of the vehicle, and the D/G direction coincides approximately with the transverse axis of the vehicle.
However, a directional control thus arranged according to the prior art does not allow control of a pure rotation of the vehicle about a central vertical axis. Indeed, such pure rotation of the vehicle about a central vertical axis (or counter-rotation) without forward or reverse movement, should be controlled by movement of the lever 2 in just the G direction, for a counter-rotation to the left, or in just the D direction, for a counter-rotation to the right, without any movement of the lever towards AV or towards AR.
However, referring to the hydraulic diagram in FIG. 1, it is seen that moving the lever 2 just to the left, for example, causes simultaneous opening of the two selectors 5 AVG and S ARG, which transmit the same hydraulic signal to the motors 6 ARG and 6 AVG, while the motors 6 ARG and 6 AVG remain inactivated. The result of this is that the forward and reverse two running components on the left-hand side are controlled simultaneously with the same intensity, and under this condition the vehicle does not move. The same would be true with movement of the lever 2 to the right.
For this single counter-rotation control, one solution could consist in crossing the feeds of the motors 6 ARG and 6 ARD, and arranging that the selector 5 ARG is then connected to the motor 6 ARD and that the selector 5 ARD is connected to the motor 6 ARG; then, in the case of, for example, moving the lever 2 to the left, the two selectors 5 AVG and 5 ARG would be simultaneously opened and would respectively control the motors 6 AVG and 6 ARD; the vehicle would then start to turn on itself in a pure rotation about its central axis. However, this rotation would then take place to the right, moreover with all the reverse controls being inverted at the same time. This would result for the driver in a difficulty from the ergonomic point of view, since the direction of the rotation performed by the vehicle would no longer correspond to the direction of movement of the lever.